Free radicals (also see Antioxidants)
Also known as free oxidizing radicals, these are oxygen molecules with an electron ‘missing’ from their outer orbit. They seek to steal electrons from any molecules with which they come into contact, thus oxidizing and destabilizing them. A single oxidizing reaction can initiate many others, resulting in a cascade of free radical formation rather like an atomic explosion.
Ultraviolet light, sunlight, X-rays or any form of high-energy ionizing radiation causes the formation of free radicals by knocking electrons out of orbit. Nuclear radiation is deadly because it generates massive numbers of free radicals. Within the body, the consumption of alcohol and tobacco greatly increases free radical activity, as do pollution, infection, overexertion, excessive amounts of iron and copper, excessive physical exercise, poor nutrition and stress.
Although free radicals are produced by a number of normal metabolic processes in the body (for instance they are by-products of normal energy production processes, liver detoxification or immune system attacks against bacteria and other micro-organisms) if the amount of oxidation which occurs in human body tissues overwhelms the body’s antioxidant capacity (its capacity to quench the oxidative reactions) this results in oxidative stress to the body’s cells, and consequent damage.
Within the cell all components are susceptible to free radical attack. In particular the lipids which constitute the cell membrane are highly susceptible, especially those lipids which contain unsaturated double bonds (see Fats for explanation of double bonds). Free radical attack on these lipids leads to the formation of poisonous peroxides, hydroperoxides and aldehydes. The membrane proteins in cells, which are involved in the transport of ions and in maintaining the balance of nutrients inside and outside cells, are also vulnerable to free radical attack. Thus free radicals can, by disrupting at least two essential components of the cell membrane, alter its structure and function, leading to membrane instability, alteration of membrane-dependent enzymes, and abnormal proportions of nutrients and fluid inside and outside cells. Impairment of the calcium-magnesium pump by free radical damage results in an increased influx of calcium into the cell, which activates arterial contraction mechanisms, reduces blood flow and can in itself destroy sensitive cells such as those in the brain and nervous system.
Free radicals also cause inflammation. For instance if they find their way into the fluid which bathes the joints in our body, (causing this fluid to lose its lubricating quality and promoting irritation) chronic inflammation and the destruction of cartilage can occur in the form of arthritis. Chronic fatigue can result from free radical damage to the membranes of mitochondria, the portion of the cell where energy is produced. Toxic compounds can be produced which result in the decreased reproduction of mitochondria and a consequent reduction in their numbers.
The superoxide (O2-) free radical is particularly common in mammals, and is a potent initiator of cell damage. One of the body’s best defences against this free radical is the antioxidant enzyme superoxide dismutase. When the superoxide radical is captured, hydrogen peroxide is created, and must be removed by another antioxidant enzyme, either catalase or glutathione peroxidase. It is thought that deficiencies of SOD and catalase are especially common in all types of arthritis. Among the flavonoids, quercetin, myricetin and rutin are the most powerful inhibitors of the superoxide ion.
Peroxides (RCOO.), which are known as ‘reactive oxygen species’ and readily disintegrate into free radicals, are formed when lipids (fats) are oxidized. Cell membranes are highly susceptible to them. Peroxides are linked with heart disease, damage to liver function, premature ageing, and skin diseases such as skin cancers, age spots and psoriasis as well as premature wrinkling. Since the liver consists of nearly 50 per cent fatty tissue, it is very vulnerable to peroxides. The antioxidant enzyme glutathione peroxidase, which consists of the amino acid glutathione and the trace element selenium, combats peroxides. The heating and reheating of cooking oils in particular promotes the generation of damaging peroxides.
Another highly reactive form of oxygen, which consists of single atoms rather than molecules, is singlet oxygen. Beta carotene and lycopene are effective scavengers of singlet oxygen. The hypochlorite ion (OCl-), formed by the oxidation of chloride ions, is also a powerful oxidizing agent, and requires adequate amounts of the amino acid taurine to control and scavenge it. Individuals with a taurine deficiency may become especially sensitive to aldehydes, bleach and chlorine, and free amino acids in their body may become toxic aldehydes. For this reason infant formula feeds should always be enriched with taurine at least to the levels found in human breast milk.
Probably the most dangerous of all free radicals is the hydroxyl (OH.) radical – the main toxin generated by exposure to radiation. Hydroxyl radicals are also formed during exercise, especially in closed rooms or in a polluted environment. Chemicals and toxins stored in our fatty tissue are released as we ‘burn off’ fat by means of exercise or dieting, and these toxins then generate the formation of hydroxyl radicals. A particularly important substance involved in combating hydroxyl radicals is an antioxidant known as melatonin. The nutrients methionine, glutathione and selenium also play an important role.
Nutrients such as vitamin E, beta carotene and flavonoids play an important part in deactivating free radicals. Without sufficient antioxidant nutrients and enzymes, oxidative stress can lead to cancers and heart disease as well as premature ageing and many associated disease processes.